Polyacrylic hydrogels, matrices for tritium liquid waste storage

Due to its high capacity for water absorption, polyacrylic acid obtained by radioinduced polymerization of acrylic acid water solution, represents an interesting alternative to HTO storage. Because changes in polymer network are produced by the β radiation emitted by tritium, radiolytic and self-radiolytic studies are required for this application. As tritium is a radioisotope with a medium half-life (12 years), simulation of radiolytic processes resulted during the storage period up to the quasitotal decay of the radionuclide is necessary. Radiometric studies using specific labelled compounds with tritium were performed. The variation of gel/sol ratio with absorbed dose, absorbed dose rate and swelling degrees were analyzed. Sol and gel fractions were determined by radiometric methods using polyacrylic hydrogels labelled with T at main chains of the polymer. Simulation of radiolytic processes was carried out using γ radiation emitted by an irradiation source of 60 Co which ensures a maximum of absorbed dose rate of 3 kGy/h. Self-radiolytic effects were investigated using labelled PAA in HTO with great radioactive concentration (37-185 GBq/mL). Influence of radioprotectors was analyzed too, but the most important parameter in radiolysis and self-radiolysis of the hydrogel system is the swelling degree. The experiments suggest as optimum for HTO storage as tritium liquid wastes a 1:30 PAA:HTO swelling degree at a radioactive concentration of HTO in the range 18. 5-37 MBq/mL. (authors)